This invention relates to an improvement in corona charging units for electrophotocopy machines and more particularly to a damage control resistor for dissipating excess corona power supply energy when an arc occurs and for quickly extinguishing the arc in order to prevent a corona wire from becoming damaged due to ion bombardment.
Corona charging units can perform a number of essential functions in the electrophotocopying process. That process includes the steps of charging a photosensitive member, exposing the charged member to an original to form a latent electrostatic image of the original on the photosensitive member, and developing the latent image. In plain paper copiers, the latent image is usually developed on the photosensitive member and the developed image is transferred to a plain paper copy sheet after which the photosensitive member is cleaned before another copy is made. Corona charging units are commonly used to initially charge the photosensitive member, to transfer the developed image to a copy sheet, and to discharge any residual charge on the photosensitive member so that it can be cleaned of untransferred developer before another copy cycle is begun.
Such corona charging units comprise an elongated fine wire that is maintained at a high electrical potential. The wire is supported on insulating blocks and is positioned between a grounded backing plate (shield) and the photosensitive member. A power supply with an output capacitance and capable of generating a high electrical (corona) potential is electrically connected to the corona wire. The power supply raises the potential of the corona wire to several thousand volts in order to generate an effective electric field in the vicinity of that portion of the photosensitive member that passes by the corona wire.
An unresolved and recurring problem for copy machine users is the breaking of corona wires. Broken corona wires must be replaced before the copy machine can resume production. Although replacement or repair of the corona wire can be easily made, temporary loss of the use of the copy machine and the repair thereof cost the user both time and money.
Until now it has been assumed that the cause of corona wire breakage was due to one or more of a number of factors that were beyond control. Such factors were believed to include arcing between the corona wire and the shield (or the photosensitive member which has a grounded, conductive backing), corrosion, vibration, and high mechanical stress in the corona wire which is tautly mounted between the insulating blocks. Regarding corona arcing, in general, it has been suggested that an external series resistor can prevent arcing. See, Leonard D. Loeb, Electrical Coronas, page 513 (1965). Moreover, there are a number of patents that describe corona charging units and show a resistor connected in series with the corona wire; see, e.g. U.S. Pat. Nos. 2,777,957 (Walkup), 2,868,989 (Haacke), 2,856,533 (Rosenthal), 3,604,925 (Snelling), 3,557,367 (Roth), 3,675,011 and 3,760,229 (both Silverberg). Furthermore, some types of commercially available power supplies have a resistor at the output terminal that is connectable in series with the corona wire of the corona charging device while other types have no such resistor. Nevertheless, experience with either type of supply has shown that corona wires will arc and break. Hence, if arcing is a cause of corona wire damage, a series resistor did not appear to be a solution to the problem.